(2S,3aS,6aS)-octahydrocyclopenta[b]pyrrole-2-carboxylic acid and esters thereof of formula (1),
wherein R1 represents hydrogen or a carboxyl-protecting group such as benzyl or tert-butyl, are key intermediates for the preparation, as for instance described in EP 79022, of the angiotensin converting enzyme (ACE) inhibitor ramipril ([2S,3aS,6aS]-1-[(2S)-2-[[(1S)-1-(ethoxycarbonyl)-3-phenylpropyl]-amino]-1-oxopropyl]octahydrocyclopenta-[b]-pyrrole-2-carboxylic acid) of formula (2).

The preparation of compounds of general formula (1) is known from various documents such as EP 79022, EP 170775, EP 1692106, EP 190224 and WO 2006/100168 and several of these approaches rely on introduction of the required chirality by hydrogenation catalyzed by high molecular weight transition state metals such as palladium, platinum and rhodium. The species that undergoes hydrogenation is a species of general formula (3).

A major drawback of the use of high molecular weight transition state metals in hydrogenation reactions is the fact that these metals are rare, expensive, highly poisonous and require dedicated recycling procedures that are economically unattractive as they usually are carried out on a relatively small scale. However, all prior art points to the belief that only high molecular weight transition state metals can afford the required degree of stereoselectivity.